The present invention generally relates to a method of printing printed material through employment of an ink-jet printing apparatus and more particularly, to a method of representing gradation in printed matter.
Conventionally, in the case where color printing is to be effected with the use of an ink-jet printing apparatus as referred to above, it is so arranged that, for a single color display or representation by either one of yellow, magenta, cyan or black, ink particles or drops in an amount equivalent to one dot are discharged onto one dot position for adhesion thereto as shown by hatched lines in FIG. 11(a), while, for a mixed color display or representation, ink drops in different colors are continuously caused to adhere onto the same dot position each by an amount equivalent to one dot as shown by intersecting hatched lines in FIG. 12(a).
A conventional ink-jet printing apparatus is adapted to effect printing by causing the ink drops discharged from its nozzle to fly onto the surface of printing paper, and therefore, the size of one dot to be displayed on the printing paper is determined by the amount of ink in the drops thus discharged. Accordingly, the amount of ink to be discharged from the nozzle is set to such a predetermined amount as will not show any gaps between dots when a solid printing is performed. Therefore, in the case of the mixed color display, ink drops in different colors are discharged onto the same dot position each by the amount equivalent to one dot, resulting in adhesion thereto of an ink amount equivalent to two dots. Thus, as shown in FIG. 12(b), more ink is soaked into the printing paper to show a larger dot display as compared with a single color dot display in FIG. 11(b). With a difference in sizes of the dots as described above, when the solid printing is effected, the larger size of the display area for the mixed color display B becomes conspicuous as compared with the size of the display area for the single color display A, and in the printing of vertical lines also, the mixed color displays B' thick in comparison with the single color displays A' as shown in FIG. 13. Accordingly, there has been such a disadvantage that the resultant color print is very unnatural due to the difference between the single color display and the mixed color display as described so far.
Meanwhile, with respect to conventional methods of representing gradations in the ink-jet printing apparatus, as illustrated in FIGS. 15(a), 15(b), 15(c) and 15(d) showing four gradations, and also in FIGS. 14(a), 14(b), 14(c), 14(d), 14(e), 14(f), 14(g), 14(h) and 14(i), there are available an area gradation representing method (1) in which the number of dots per unit area is adapted to be variable according to the degrees of gradations, and another method (2) employed for an on-demand type printer, etc. and so arranged that, by controlling a driving voltage for the ink discharging or width of pulses according to the gradations, with the number of dots held constant, the size of the dot is varied to correspond to the gradation through variation of the amount of ink for one dot.
However, in the above method (1), if it is intended to suppress the dot density to be the same degree as in the case of a non-gradation, the numbers of dots in the longitudinal direction and vertical direction must be doubled respectively for the four gradations, and in this case, in order to print the same area as compared with that during the non-gradation over the same period of time, it is necessary double the running speed of a head, and also to increase the ink discharge driving frequency by a factor of four. Although it is possible to increase the driving frequency by a factor of four, there are various problems related to the doubling of the head running speed, such as a violent movement of ink within an ink tank, an increased size of the head driving motor, etc. Furthermore, as is seen from FIGS. 14 and 15, due to non-uniformity in the distribution of dots, the gradation linearity can not be regarded as favorable, resulting in an unnatural representation of gradations.
Meanwhile, according to the above conventional method (2), although the gradation linearity is better than in method (1), there are problems with respect to the performance of the nozzle, discharged ink particles, and stability of the nozzle in functioning, owing to the fact that the amount of ink continuously discharged from the nozzle is increased per one jetting as the degree of gradation increases, with a simultaneous prolongation of the ink discharging time, and thus, method (2) has not been put into actual application as yet, since there is a limit to the gradation representation in multi-stages (FIG. 16).